1. Field of the Invention
The present invention relates to ceramic capacitors, particularly to monolithic ceramic capacitors having thin dielectric ceramic layers, and relates to methods for making the same.
2. Description of the Related Art
Ceramic capacitors, particularly monolithic ceramic capacitors, are typically produced as follows. Dielectric sheets, each primarily composed of, for example, BaTiO.sub.3, are prepared. An electrode material for an internal electrode is applied to a surface of each dielectric sheet. The dielectric sheets provided with electrode materials are laminated, are thermally compressed, and are sintered at 1,250 to 1,350.degree. C. in an ambient atmosphere to form a dielectric ceramic having the internal electrodes. External electrodes connecting to the internal electrodes are provided by baking on both sides of the dielectric ceramic to form a monolithic ceramic capacitor.
Thus, the internal electrode material must satisfy the following conditions.
(A) Since the dielectric sheets and the internal electrode material are simultaneously sintered, the internal electrode material must have a melting point which is higher than the temperature at which the dielectric sheet material is sintered.
(B) The internal electrode material must not be oxidized and must not react with the dielectric sheet material in an oxidative high-temperature atmosphere.
As internal electrode materials satisfying such conditions, noble metals, such as platinum, gold, palladium and silver-palladium alloys, have been used since these noble metals exhibit superior properties when used as internal electrodes. These noble metals, however, are expensive, and this is a major factor increasing production costs of monolithic ceramic capacitors.
Other than noble metals, base metals having high melting points are, for example, Ni, Fe, Co, W, and Mo. These base metals, however, are readily oxidized in high-temperature oxidizing atmospheres and thus are not suitable for electrodes. When these base metals are used as internal electrodes of monolithic ceramic capacitors, the composite must be sintered in neutral or reducing atmospheres. Conventional dielectric ceramic materials, however, are significantly reduced during sintering in such neutral or reducing atmospheres and are thus converted to semiconductor materials.
In order to solve this problem, Japanese Examined Patent Application Publication No. 57-42588 discloses a dielectric material composed of a barium titanate solid solution in which the ratio of the barium site to the titanium site is stoichiometrically excessive. In addition, Japanese Unexamined Patent Application Publication No. 61-101459 discloses a dielectric material composed of a barium titanate solid solution containing an oxide of a rare earth element, such as La, Nd, Sm, Dy or Y.
Known dielectric materials exhibiting a small change in dielectric constant with temperature include, for example, a BaTiO.sub.3 --CaZrO.sub.3 --MnO--MgO based composition disclosed in Japanese Unexamined Patent Application Publication No. 62-25422, and a BaTiO.sub.3 --(Mg,Zn,Sr,Ca)O--B.sub.2 O.sub.3 --SiO.sub.2 based composition disclosed in Japanese Examined Patent Application Publication No. 61-14611. Since these dielectric materials are not converted into semiconductor materials during sintering in reducing atmospheres, base metals such as nickel can be used as internal electrodes in the production of monolithic ceramic capacitors.
In recent years, miniaturization of electronic components is making rapid progress as electronic technologies develop, and further miniaturization and larger capacitances are required in monolithic ceramic capacitors. Thus, development of higher dielectric constant dielectric materials and reduced thickness dielectric ceramic layers is rapidly progressing. Current dielectric materials are, therefore, required to have high dielectric constants, small changes in dielectric constant with temperature and high reliability.
Although the dielectric materials disclosed in Japanese Examined Patent Application Publication No. 57-42588 and Japanese Unexamined Patent Application Publication No. 61-101459 have large dielectric constants, segregation of rare earth elements readily occurs in dielectric ceramic matrices. Such segregation of rare earth elements causes significantly reduced reliability of thin dielectric ceramic layers having a thickness of 10 .mu.m or less.
The dielectric material disclosed in Japanese Unexamined Patent Application Publication No. 62-256422 has a relatively large dielectric constant and changes in dielectric constant with temperature are small. In this material, however, CaZrO.sub.3, and CaTiO.sub.3 are formed during sintering and readily form secondary phases together with Mn. Thus, this dielectric material exhibits less reliability at high temperatures.
When secondary phases are formed by segregation of one or more elements in the primary phase of a dielectric ceramic layer, reliability is significantly reduced in a monolithic ceramic capacitor comprising such dielectric ceramic layers having a thickness of 10 .mu.m or less. When the secondary phases exhibit low resistance to reduction, sintering in a reducing atmosphere causes further decrease in the reliability of the monolithic ceramic capacitor.